TEXAS Republicans have a beef.They say that their numbers have increased because of population gains to the point where they would be able to win half the state's elected seats - if districts were drawn fairly. But the state is controlled by Democrats. Gov. Ann Richards is one. And Democrats dominate the Texas House of Representatives 93 to 57 and the Senate 22 to 9. Democrats also hold 19 of the 27 existing Texas seats in the US Congress and are expected to win the three new districts. One has been designed to seat a black from Dallas. Another will elect a Latino from south of San Antonio. The last will elect a Latino from Houston, if not in 1992 then certainly sometime this decade, says Robert Slagle, state chairman of the Democratic Party. Redistricting "was a Democrat-controlled process throughout," says James Duncan, a redistricting consultant to the state Republican Party. The new district boundaries are a product of "blatant political gerrymandering," he says. Texas has 17.5 million residents, up from 14 million a decade ago. Voters don't register by party in Texas, but some 1.4 million people cast votes in the 1990 Republican primary, he says. Only 265,000 did so in 1982. Only 1.7 million people cast ballots as Democrats last year, he says. And in statewide contests, Republican candidates have taken as much as 55 percent of the vote, Mr. Duncan says. Also, conservative Democrats have been switching parties. Republicans have filed lawsuits intending to overturn the Democrat-drawn state and federal election districts. Duncan says the Democrats "packed" as many Republicans as they could into as few districts as possible. The lawsuits could be concluded in time for next spring's primaries. The enlarged Texas delegation in the US House of Representatives is expected to use its added clout to watch out for the interests of the oil and gas industry. The 30 congressmen, including three from new districts, also are expected to resist any attempts to make Texas bear a disproportionate share of the cost of the savings-and-loan crisis, as some have suggested it should, based on the allegation that this state caused the problem.